👀 Views: 535 💬 Answers: 1 📅 Created: 2025-08-25

jest typescript unit-testing TypeScript

I'm performance testing and I'm struggling to unit test a TypeScript utility function that processes an array of numbers and returns their string representations. Despite having written tests that should cover the basic functionality, I'm working with the behavior 'want to read property 'toString' of undefined' when running my tests. Here's the function I'm trying to test: ```typescript // src/utils/numberToString.ts export const numberToString = (numbers: (number | undefined)[]): string[] => { return numbers.map(num => num.toString()); }; ``` And here are the relevant tests: ```typescript // src/utils/numberToString.test.ts import { numberToString } from './numberToString'; describe('numberToString', () => { it('should convert an array of numbers to an array of strings', () => { expect(numberToString([1, 2, 3])).toEqual(['1', '2', '3']); }); it('should handle undefined values gracefully', () => { expect(numberToString([1, undefined, 3])).toEqual(['1', 'undefined', '3']); }); }); ``` In the second test, I expected that the `undefined` value would be converted to the string 'undefined', but instead, I'm getting the behavior when trying to call `toString()` on `undefined`. I tried adding a check for `undefined` within my `numberToString` function, but it didn’t resolve the scenario. Here's my modified function: ```typescript // src/utils/numberToString.ts export const numberToString = (numbers: (number | undefined)[]): string[] => { return numbers.map(num => (num !== undefined ? num.toString() : 'undefined')); }; ``` However, this change does not help either, and the tests still unexpected result with the same behavior. I’ve ensured that my Jest configuration is set up correctly and that TypeScript is compiling without issues. I'm using Jest version 27.0 and TypeScript version 4.5. Please advise on how to handle this scenario correctly in my unit tests. What am I missing?